Process for preparing bitumen/rubber compositions

ABSTRACT

In methods of preparing asphalt and elastomeric polymer compositions, it has been discovered that for a given crosslinker or vulcanizing agent there is an optimum crosslinking temperature to give a composition that has top and bottom softening points that are close. That is, rubber/asphalt compatibility is improved, where the crosslinking is performed within the optimum temperature range.

FIELD OF THE INVENTION

[0001] The present invention is related to hydrocarbon-based binders,such as bitumens, asphalts and tars, modified with elastomers, andincluding a vulcanized stage, which are particularly useful asindustrial coatings and road bitumens, or the like. It relates moreparticularly to processes for obtaining vulcanized compositions based onbitumens and on styrene/butadiene copolymers that have close top andbottom softening points.

BACKGROUND OF THE INVENTION

[0002] The use of bitumen (asphalt) compositions in preparing aggregatecompositions (including, but not just limited to, bitumen and rock)useful as road paving material is complicated by at least three factors,each of which imposes a serious challenge to providing an acceptableproduct. First, the bitumen compositions must meet certain performancecriteria or specifications in order to be considered useful for roadpaving. For example, to ensure acceptable performance, state and federalagencies issue specifications for various bitumen applications includingspecifications for use as road pavement. Current Federal HighwayAdministration specifications require a bitumen (asphalt) product tomeet defined parameters relating to properties such as viscosity,stiffness, penetration, toughness, tenacity and ductility. Each of theseparameters define a critical feature of the bitumen composition, andcompositions failing to meet one or more of these parameters will renderthat composition unacceptable for use as road pavement material.

[0003] Conventional bitumen compositions frequently cannot meet all ofthe requirements of a particular specification simultaneously and, ifthese specifications are not met, damage to the resulting road canoccur, including, but not necessarily limited to, permanent deformation,thermally induced cracking and flexural fatigue. This damage greatlyreduces the effective life of paved roads.

[0004] In this regard, it has long been recognized that the propertiesof conventional bitumen compositions can be modified by the addition ofother substances, such as polymers. A wide variety of polymers have beenused as additives in bitumen compositions. For example, copolymersderived from styrene and conjugated dienes, such as butadiene orisoprene, are particularly useful, since these copolymers have goodsolubility in bitumen compositions and the resulting modified-bitumencompositions have good rheological properties.

[0005] It is also known that the stability of polymer-bitumencompositions can be increased by the addition of crosslinking agents(vulcanizing agents) such as sulfur, frequently in the form of elementalsulfur. It is believed that the sulfur chemically couples the polymerand the bitumen through sulfide and/or polysulfide bonds. The additionof extraneous sulfur is required to produce the improved stability, eventhough bitumens naturally contain varying amounts of native sulfur.

[0006] Thus, there is known a process for preparing a bitumen-polymercomposition consisting of mixing a bitumen, at 266-446° F. (130-230°C.), with 2 to 20% by weight of a block or random copolymer, having anaverage molecular weight between 30,000 and 300,000. The resultingmixture is stirred for at least two hours, and then 0.1 to 3% by weightof sulfur relative to the bitumen is added and the mixture agitated forat least 20 minutes. The quantity of added sulfur cited in this patentis 0.1 to 1.5% by weight with respect to the bitumen. The resultingbitumen-polymer composition is used for road-coating, industrialcoating, or other industrial applications.

[0007] Similarly, there is also known an asphalt (bitumen) polymercomposition obtained by hot-blending asphalt with 0.1 to 1.5% by weightof elemental sulfur and 2 to 7% by weight of a natural or syntheticrubber, which can be a linear butadiene/styrene copolymer. A process isadditionally known for preparing a rubber-modified bitumen by blendingrubber, either natural or synthetic, such as styrene/butadiene rubber,with bitumen at 293-365° F. (145-185° C.), in an amount up to 10% byweight based on the bitumen, then adjusting the temperature to 257-320°F. (125-160° C.), and intimately blending into the mix an amount ofsulfur such that the weight ratio of sulfur to rubber is between 0.01and 0.9. A catalytic quantity of a vulcanization-accelerator is thenadded to effect vulcanization. A critical nature of the sulfur to rubberratio is sometimes reported, for instance that weight ratios of sulfurto rubber of less than 0.01 gives modified bitumen of inferior quality.

[0008] The second factor complicating the use of bitumen compositionsconcerns the viscosity stability of such compositions under storageconditions. In this regard, bitumen compositions are frequently storedfor up to 7 days or more before being used and, in some cases, theviscosity of the composition can increase so much that the bitumencomposition is unusable for its intended purpose. On the other hand, astorage stable bitumen composition would provide for only minimalviscosity increases and, accordingly, after storage it can still beemployed for its intended purpose.

[0009] Asphaltic concrete, typically including asphalt and aggregate,asphalt compositions for resurfacing asphaltic concrete, and similarasphalt compositions must exhibit a certain number of specificmechanical properties to enable their use in various fields ofapplication, especially when the asphalts are used as binders forsuperficial coats (road surfacing), as asphalt emulsions, or inindustrial applications. (The term “asphalt” is used hereininterchangeably with “bitumen.” Asphaltic concrete is asphalt used as abinder with appropriate aggregate added, typically for use in roadways.)The use of asphalt or asphalt emulsion binders either in maintenancefacings as a surface coat or as a very thin bituminous mix, or as athicker structural layer of bituminous mix in asphaltic concrete, isenhanced if these binders possess the requisite properties such asdesirable levels of elasticity and plasticity.

[0010] As noted, various polymers have been added to asphalts to improvephysical and mechanical performance properties. Polymer-modifiedasphalts (PMAs) are routinely used in the road construction/maintenanceand roofing industries. Conventional asphalts often do not retainsufficient elasticity in use and, also, exhibit a plasticity range thatis too narrow for use in many modern applications such as roadconstruction. It is known that the characteristics of road asphalts andthe like can be greatly improved by incorporating into them anelastomeric-type polymer which may be one such as butyl, polybutadiene,polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer,polyacrylate, polymethacrylate, polychloroprene, polynorbornene,ethylene/propylene/diene (EPDM) terpolymer and advantageously a randomor block copolymer of styrene and a conjugated diene. The modifiedasphalts thus obtained commonly are referred to variously asbitumen/polymer binders or asphalt/polymer mixes. Modified asphalts andasphalt emulsions typically are produced utilizing styrene/butadienebased polymers, and typically have raised softening point, increasedviscoelasticity, enhanced force under strain, enhanced strain recovery,and improved low temperature strain characteristics as compared withnon-modified asphalts and asphalt emulsions.

[0011] The bituminous binders, even of the bitumen/polymer type, whichare presently employed in road applications often do not have theoptimum characteristics at low enough polymer concentrations toconsistently meet the increasing structural and workability requirementsimposed on roadway structures and their construction. In order toachieve a given level of modified asphalt performance, various polymersare added at some prescribed concentration.

[0012] Current practice is to add the desired level of a single polymer,sometimes along with a reactant that promotes cross-linking of thepolymer molecules until the desired asphalt properties are met. Thisreactant typically is sulfur in a form suitable for reacting.

[0013] However, the cost of the polymer adds significantly to theoverall cost of the resulting asphalt/polymer mix. Thus, cost factorsweigh in the ability to meet the above criteria for various asphaltmixes. In addition, at increasing levels of polymer concentration, theworking viscosity of the asphalt mix becomes excessively great andseparation of the asphalt and polymer may occur.

[0014] It is common in the preparation of polymer-modified asphalts toinclude activators and accelerators to make the crosslinking reactionproceed faster. Zinc oxide (ZnO) is a conventional activator, andmercaptobenzothiazole (MBT) is a conventional accelerator. ZnO is alsosometimes used to control the tendency of the polymer to gel. The zincsalt of mercaptobenzothiazole (ZMBT) combines features of both of theseconventional additives.

[0015] In view of the above, bitumen compositions, which simultaneouslymeet the performance criteria required for road paving, and which use analternative activator to the relatively costly ZnO would beadvantageous. Additionally, having available a variety of differentactivators for bitumen compositions would provide versatility. Inpreparing the composition, significant mixing is needed to insure theuniform addition of both the polymer and any crosslinking agents,accelerators or activators. The crosslinking agents and other agents areusually added as a dry powder and mixed with the asphalt compositions.

[0016] One of the methods commonly utilized in the industry tostandardize the measure or degree of compatibility of the rubber withthe asphalt is referred to as the compatibility test. The test comprisesthe mixing of the rubber and asphalt with all the applicable additives,such as the crosslinking agents. The mixture is placed in tubes, usuallymade of aluminum or similar material, referred to as cigar tubes ortoothpaste tubes. These tubes are about one inch in diameter and aboutfifteen centimeters deep. The mixture is placed in an oven heated to atemperature of about 162° C. (320° F.). This temperature isrepresentative of the most commonly used asphalt storage temperature.After the required period of time, most commonly twenty-four (24) hours,the tubes are transferred from the oven to a freezer and cooled down tosolidify. The tubes are kept in the vertical position. After coolingdown, the tubes are cut into thirds; three equal sections. Thering-and-ball softening point of the top one third is compared to thesoftening point of the bottom section. This test gives an indication ofthe separation or compatibility of the rubber within the asphalt. Therubber would have the tendency to separate to the top. The lower thedifference in softening point between the top and bottom sections, themore compatible are the rubber and asphalt. In today's environment, moststates require a difference of 4° F. (2° C.) or less to consider theasphalt/rubber composition as compatible. Few standards allow a higherdifference. The twenty-four hour test is used as a common comparisonpoint.

[0017] As can be seen from the above, methods are known to improve themixing of asphalt and polymer compositions. The needed elements for thecommercial success of any such process include keeping the process assimple as possible, reducing the cost of the ingredients, and utilizingavailable asphalt cuts from a refinery without having to blend in morevaluable fractions. In addition, the resulting asphalt composition mustmeet the above-mentioned governmental physical properties andenvironmental concerns. Thus, it is a goal of the industry to maintainor reduce the cost of the polymers and crosslinking agents added to theasphalt without sacrificing any of the other elements and improving theproperties of the asphalt and polymer compositions as much as possible.

SUMMARY OF THE INVENTION

[0018] There is provided, in one form, a method for preparing asphaltand polymer compositions involving first heating a mixture of asphaltand an elastomeric polymer to within an optimum crosslinking temperaturerange, and then adding a crosslinker to the mixture, where thecrosslinker may be a sulfur-containing derivative, elemental sulfur andmixtures thereof. The optimum crosslinking temperature range is thatwhere the resulting asphalt/polymer composition has a difference betweenthe top and bottom softening points of 10° C. or less.

[0019] In another embodiment of the invention, there are providedasphalt and polymer compositions made by the process described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph of the compatibility of the asphalt/polymercompositions of this invention plotted in terms of separation as afunction of crosslinking temperature;

[0021]FIG. 2 is a graph illustrating the relationship betweenpenetration and the crosslinking temperature; and

[0022]FIG. 3 is a graph plotting Brookfield viscosity as a function ofcrosslinking temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0023] It has been discovered that improvements in rubber/asphaltcompatibility may be obtained by crosslinking with certain crosslinkersat temperatures higher than those normally employed. When particularcrosslinkers are used to crosslink mixtures of asphalt and elastomericpolymers there is an optimum crosslinking temperature range at which thecrosslinker is added and reacted with the mixture. This optimumcrosslinking temperature range gives resulting crosslinked compositionswith small differences between the top and bottom softening points. Inone non-limiting embodiment, the difference between these two points isbroadly 20° C. or less, in one non-limiting embodiment 10° C. or less,alternately can be 4° C. or less, and can be 2° C. or less. Thisinvention may be considered as a potential alternative to the use ofresins or other techniques to reduce separation.

[0024] Care must be taken in not subjecting the asphalt/polymercomposition to elevated temperatures for too long to avoid thermaldegradation of the polymer. Thus, the mixture of asphalt and elastomericpolymer can be maintained within the optimum crosslinking temperaturerange for a minimal period of time that is typically empiricallydetermined. However, in one non-limiting embodiment of the invention,the mixture is kept within the optimum crosslinking temperature rangefor a time period ranging from about 30 to about 120 minutes, such asfor example, 60 minutes.

[0025] As used herein, the term “bitumen” (sometimes referred to as“asphalt”) refers to all types of bitumens, including those that occurin nature and those obtained in petroleum processing. The choice ofbitumen will depend essentially on the particular application intendedfor the resulting bitumen composition. Bitumens that can be used canhave an initial viscosity at 140° F. (60° C.) of 600 to 3000 poise (60to 300 Pa-s) depending on the grade of asphalt desired. The initialpenetration range (ASTM D5) of the base bitumen at 77° F. (25° C.) is 20to 320 dmm, and can be 50 to 150 dmm, when the intended use of thecopolymer-bitumen composition is road paving. Bitumens that do notcontain any copolymer, sulfur, etc., are sometimes referred to herein asa “base bitumen.”

[0026] “Elastomeric Polymers” are natural or synthetic rubbers andinclude, but are not necessarily limited to, butyl, polybutadiene,polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer,polyacrylate, polymethacrylate, polychloroprene, polynorbornene,ethylene/propylene/diene (EPDM) terpolymer and advantageously a randomor block copolymer of a vinyl aromatic compound, e.g. styrene, andconjugated dienes. In one non-limiting embodiment of the invention,styrene/conjugated diene block copolymers may be used that are linear,radial, or multi-branched. Styrene/butadiene and styrene/isoprenecopolymers having an average molecular weight of between 30,000 and300,000 have been found to be particularly useful in the presentinvention.

[0027] “Conjugated dienes” refer to alkene compounds having 2 or moresites of unsaturation wherein a second site of unsaturation isconjugated to a first site of unsaturation, i.e., the first carbon atomof the second site of unsaturation is gamma (at carbon atom 3) relativeto the first carbon atom of the first site of unsaturation. Conjugateddienes include, by way of non-limiting example, butadiene, isoprene,1,3-pentadiene, and the like.

[0028] “Block copolymers of styrene and conjugated-dienes” refer tocopolymers of styrene and conjugated-dienes having a linear or radial,tri-block structure consisting of styrene-conjugated diene-styrene blockunits that are copolymers are represented by the formula:

S_(x)-D_(y)-S_(z)

[0029] where D is a conjugated-diene, S is styrene, and x, y and z areintegers such that the number average molecular weight of the copolymeris from about 30,000 to about 300,000. These copolymers are well knownto those skilled in the art and are either commercially available or canbe prepared from methods known in the art. Such tri-block copolymers maybe derived from styrene and a conjugated-diene, wherein theconjugated-diene is butadiene or isoprene. Such copolymers may contain15 to 50 percent by weight copolymer units derived from styrene,alternatively may contain 20 to 35 percent derived from styrene, andthen again may contain 28 to 31 percent derived from styrene, theremainder being derived from the conjugated diene. These copolymers mayhave a number average molecular weight range between 50,000 and 200,000,and alternatively have a number average molecular weight range between80,000 and 180,000. The copolymer can employ a minimal amount ofhydrocarbon solvent in order to facilitate handling. Examples ofsuitable solvents include plasticizer solvent that is a non-volatilearomatic oil. However, when the hydrocarbon solvent is a volatilesolvent (as defined above), care should be taken to ensure that theamount of solvent contained in the final bitumen composition is lessthan about 3.5 weight percent.

[0030] The term “sulfur” is defined herein as elemental sulfur in any ofits physical forms, whereas the term “sulfur-containing derivative”includes any sulfur-donating compound, but not elemental sulfur.Sulfur-donating compounds are well known in the art and include variousorganic compositions or compounds that generate sulfur under the mixingor preparation conditions of the present invention. In one non-limitingembodiment, the elemental sulfur is in powder form known as flowers ofsulfur. Other sulfur-containing derivatives or species that can be usedin the invention include, but are not necessarily limited tomercaptobenzothiazole, thiurams, and the like, and combinations thereof.In another non-limiting embodiment of the invention, the sulfur ispresent in an amount ranging from about 0.06% to about 0.3 wt. % basedon the asphalt, abd alternatively is present in an amount from about0.08 to about 0.2 wt. %.

[0031] Acceptable crosslinkers, in one non-limiting embodiment of theinvention, are thiuram polysulfides. In another non-limiting embodimentof the invention, the thiuram polysulfides have the formula:

[0032] where R¹ and R² are the same or different alkyl substituentshaving from 1 to 4 carbon atoms, and wherein M is a metal selected fromzinc, barium or copper, and n is 0 or 1. In another non-limitingembodiment of the invention, a crosslinking temperature range forthiuram polysulfides of formula (I) is above 180° C. (356° F.),alternatively, the crosslinking temperature range may be between about185 and about 190° C. (365-374° F.). In one non-limiting embodiment ofthe invention, the optimal crosslinking temperature range for aparticular crosslinker is determined empirically. In anothernon-limiting embodiment of the invention, the optimal crosslinkingtemperature range is 20° C. wide, in one non-limiting embodiment of theinvention 10° C. wide, in another non-limiting embodiment 4° C. wide,and in yet another non-limiting embodiment of the invention 5° C. wideor less.

[0033] In still another non-limiting embodiment of the invention, thesulfur-containing derivative excludes added elemental sulfur, per se.Alternatively, the asphalt and elastomeric polymer mixture may containadded elemental sulfur, but the crosslinking is conducted at atemperature different from the optimum crosslinking temperature forelemental sulfur, per se.

[0034] The term “desired Rheological Properties” refers primarily to theSUPERPAVE asphalt binder specification designated by AASHTO as SP-1.Additional asphalt specifications can include viscosity at 140° F. (60°C.) of from 1600 to 4000 poise (160-400 Pa-s) before aging; a toughnessof at least 110 inch-pound (127 cm-kilograms) before aging; a tenacityof at least 75 inch-pound (86.6 cm-kilograms) before aging; and aductility of at least 25 cm at 39.2° F. (4° C.) at 5 cm/min. pull rateafter aging.

[0035] Viscosity measurements are made by using ASTM test method D2171.Ductility measurements are made by using ASTM test method D113.Toughness and tenacity measurements are made by a Benson Method ofToughness and Tenacity, run at 20 inches/minute (50.8 cm/minute) pullrate with a ⅛ inch (2.22 cm) diameter ball.

[0036] By “storage stable viscosity” it is meant that the bitumencomposition shows no evidence of skinning, settlement, gelation, orgraininess and that the viscosity of the composition does not increaseby a factor of four or more during storage at 325±0.5° F. (163±2.8° C.)for seven days. In one non-limiting embodiment of the invention, theviscosity does not increase by a factor of two or more during storage at325° F. (163° C.) for seven days. In another non-limiting embodiment ofthe invention, the viscosity increases less than 50% during seven daysof storage at 325° F. (163° C.). A substantial increase in the viscosityof the bitumen composition during storage is not desirable due to theresulting difficulties in handling the composition and in meetingproduct specifications at the time of sale and use.

[0037] The term “aggregate” refers to rock and similar material added tothe bitumen composition to provide an aggregate composition suitable forpaving roads. Typically, the aggregate employed is rock indigenous tothe area where the bitumen composition is produced. Suitable aggregateincludes granite, basalt, limestone, and the like.

[0038] As used herein, the term “asphalt cement” refers to any of avariety of substantially solid or semi-solid materials at roomtemperature that gradually liquify when heated. Its predominantconstituents are bitumens, which may be naturally occurring or obtainedas the residue of refining processing. The asphalt terms used herein arewell known to those skilled in the art. For an explanation of theseterms, reference is made to the booklet SUPERPAVE Series No. 1 (SP-1),1997 printing, published by the Asphalt Institute (Research Park Drive,P.O. Box 14052, Lexington, Ky. 40512-4052), which is hereby incorporatedby reference in its entirety. For example, Chapter 2 provides anexplanation of the test equipment, terms, and purposes. Rolling ThinFilm Oven (RTFO) and Pressure Aging Vessel (PAV) are used to simulatebinder aging (hardening) characteristics. Dynamic Shear Rheometers (DSR)are used to measure binder properties at high and intermediatetemperatures. This is used to predict permanent deformation or ruttingand fatigue cracking. Bending Beam Rheometers (BBRs) are used to measurebinder properties at low temperatures. These values predict thermal orlow temperature cracking. The procedures for these experiments are alsodescribed in the above-referenced SUPERPAVE booklet.

[0039] Asphalt grading is given in accordance with accepted standards inthe industry as discussed in the above-referenced Asphalt Institutebooklet. For example, pages 62-65 of the booklet include a tableentitled Performance Graded Asphalt Binder Specifications. The asphaltcompositions are given performance grades, for example, PG 64-22. Thefirst number, 64, represents the average 7-day maximum pavement designtemperature in ° C. The second number, −22, represents the minimumpavement design temperature in ° C. Other requirements of each grade areshown in the table. For example, the maximum value for the PAV-DSR test(° C.) for PG 64-22 is 25° C.

[0040] In accordance with one non-limiting embodiment of the presentinvention, an asphalt composition is prepared by adding the asphalt orbitumen to a mixing tank that has stirring means. The asphalt is addedand stirred at elevated temperatures. Stirring temperatures depend onthe viscosity of the asphalt and can range up to 500° F. (260° C.).Asphalt products from refinery operations are well known in the art. Forexample, asphalts typically used for this process are obtained from deepvacuum distillation of crude oil to obtain a bottom product of thedesired viscosity or from a solvent deasphalting process that yields ademetallized oil, a resin fraction and an asphaltene fraction. Somerefinery units do not have a resin fraction. These materials or othercompatible oils of greater than 450° F. (232° C.) flash point may beblended to obtain the desired viscosity asphalt.

[0041] Rubbers, elastomeric polymers, or thermoplastic elastomerssuitable for this application are well known in the art as describedabove. For example, FINAPRENE® products available from AtofinaPetrochemicals Inc. are suitable for the applications of the presentinvention. This example is not limiting for the inventive technologythat can be applied to any similar elastomeric product particularlythose produced from styrene and butadiene.

[0042] Various additives suitable for the purposes of this inventioninclude, but are not necessarily limited to, known and futureaccelerators, activators, and the like. A variety of accelerators may beused in conjunction with this invention, including, but not limited to,dithiocarbamates and benzothiazoles.

[0043] The methods and compositions of this invention will be furtherillustrated with respect to particular Examples that are only intendedto more fully illuminate the invention and not limit it.

EXAMPLES 1-13

[0044] The base asphalt used was a PG 64-22. It was made by blending anasphaltenes sample designated 97-116 with a flux oil designated 97-134.The Triflux 250 flux oil was produced from Boscan crude.

[0045] This blend initially showed severe incompatibility with F 411 SBSrubber. The top and bottom softening points of aged samples differed byas much as 100° F. (56° C.).

[0046] The asphaltene/flux oil blend was preheated and held at thespecified mixing temperature for 30 minutes with an oil bath. The mixingtemperature was that of the asphalt as measured with a hand-heldthermocouple. The rubber was added and mixed with a Silverson high shearlab mixer until a smooth homogeneous mixture was obtained (approximately30-45 minutes).

[0047] The high shear mixer was replaced with a low shear prop mixer.The temperature of oil bath was then adjusted to bring theasphalt/rubber mixture to the proper crosslinking temperature andallowed to stabilize. The crosslinking additives were slowly added. Lowshear mixing was continued for 90 minutes. The samples were then placedin a 325° F. (163° C.) oven overnight. On removal, the samples werestirred for a few minutes with a low shear mixer to insure homogeneity.The samples were then poured for separation and other testing.

[0048] The results are shown in Table I and a plot of separation versuscrosslinking temperature is graphed in FIG. 1. It may be seen that adramatic improvement in separation was obtained at an optimalcrosslinking temperature of about 370° F. (188° C.). TABLE I Evaluationof Crosslinking Temperature in Asphalt Crosslinking Systems Ex.:Ingredients 1 2 3 4 5 6 7 8 9 10 11 12 Flux Oil (97-134) Wt %  80  80 80  80  80  80  80  80  80  80  80  80 Asphaltenes Wt %  20  20  20  20 20  20  20  20  20  20  20  20 (97-116) F 401 Wt %  4  2.67  1.33 F 502Wt %  4  1.33  2.67 F 411 Wt %  4  3  3  3  3  3 Kraton 1101 Wt %  4Kraton 1116 Wt %  4 Sulfur Wt %  0.05  0.05  0.05  0.05  0.05  0.05 0.05  0.05  0.05  0.05  0.05  0.05 Methyl Zimate Wt %  0.05  0.05  0.05 0.05  0.05  0.05  0.05  0.05  0.05  0.05  0.05  0.05 Mixing ° F. 350350 350 350 350 350 350 370 370 370 370 370 temperature (° C.) (177)(177) (177) (177) (177) (177) (177) (188) (188) (188) (188) (188)Crosslinking ° F. 320 320 320 320 320 320 320 370 340 355 390 380temperature (° C.) (160) (160) (160) (160) (160) (160) (160) (188) (171)(179) (199) (193) Compatibility 24 hrs - 325° F. (163° C.) Top ° F.163.7 162.2 236.2 174.7 171.3 171.0 169.0 139.8 190.5 165.0 189.1 175.3(° C.)  (73.2)  (72.3) (113.4)  (79.3)  (77.4)  (77.2)  (76.1)  (59.9) (88.1)  (73.9)  (87.3)  (79.6) Bottom ° F. 142.2 149.7 141.5 140.5140.8 138.4 144.0 140.8 140.1 140.5 138.5 137.7 (° C.)  (61.2)  (65.3) (60.8)  (60.3)  (60.4)  (59.1)  (62.2)  (60.4)  (60.1)  (60.3)  (59.1) (58.7) Delta ° F.  21.5  12.5  94.7  34.2  30.5  32.6  25.0  −1.0  50.4 24.5  50.6  37.6 (separation) (° C.)  (12.0)  (7.0)  (52.6)  (19.0) (17.0)  (18.1)  (13.9)  (−0.5)  (28.0)  (13.6)  (28.2)  (20.9)

[0049] F 401 is a Finaprene SBS Rubber available from AtofinaPetrochemical.

[0050] F 502 is a Finaprene SBS Rubber available from AtofinaPetrochemical.

[0051] F 411 is a Finaprene SBS Rubber available from AtofinaPetrochemical.

[0052] Kraton 1101 is a SBS Rubber available from Kraton Polymers.

[0053] Kraton 1116 is a SBS Rubber available from Kraton Polymers

[0054] Methyl Zimate® is a trademarked name for zincdimethyidithiocarbamate available from R. T. Vanderbilt, Inc. and is athiuram polysulfide within the scope of this invention.

[0055] One potential explanation of the phenomenon illustrated in FIG. 1is that as the crosslinking temperature is increased, the reactionbecomes less selective with interspecies links formed between the rubberand the asphaltenes. At still higher temperatures, more crosslinkingbetween asphaltene molecules occurs which again makes them incompatiblewith rubber. However, the inventor does not want the invention to belimited by any particular theory, explanation or supposed mechanism.

[0056] Two other properties of the crosslinked asphalt were measured asa part of the study: penetration and viscosity at 350° F. (177° C.). Asshown in FIG. 2, there was no significant trend in penetration atdifferent crosslinking temperatures. The viscosity decreased as thecrosslinking temperature increased as shown in FIG. 3, most likely dueto the improved compatibility.

[0057] One concern of raising the crosslinking temperature to 370° F.(188° C.) is the increased risk of excessive thermal crosslinkingoccurring that may cause the asphalt to be unusable (high viscositygel). Thus, a balance needs to be achieved in choosing a crosslinkingtemperature between obtaining good separation and minimizing thermalcrosslinking.

[0058] The data for Examples 14 through 18 presented in Table II belowdemonstrates that crosslinking may be achieved at elevated temperatures(380° F.; 193° C.) using the method of this invention without causingthe asphalt/polymer composition to gel, as shown for Examples 16 and 18.TABLE II Evaluation of Crosslinking Temperature in Asphalt CrosslinkingSystems Example Component Units 13 14 15 16 17 SBS Finaprene 411 Wt % 22 2 SBS Kraton 1184 Wt % 2 2 Sulfur Wt % 0.03 0.03 0.03 0.03 0.03 Zincoxide Wt % 0.02 0.02 0.02 0.02 0.02 MBT Wt % 0.02 0.02 0.02 0.02 0.02Mixing ° F. 325 360 380 360 380 Temperature (° C.) (163) (182) (193)(182) (193) Blend result gelled gelled no gel gelled no gel

[0059] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof, and has beendemonstrated as effective in providing methods for preparing asphalt andpolymer compositions with optimized separation between the top andbottom softening points. However, it will be evident that variousmodifications and changes can be made to the method without departingfrom the broader spirit or scope of the invention as set forth in theappended claims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, specificcombinations or amounts of asphalt, polymer, crosslinker, activator,accelerator, and other components falling within the claimed parameters,but not specifically identified or tried in a particular PMA system, areanticipated and expected to be within the scope of this invention.Specifically, the method and discovery of the invention are expected towork with crosslinkers other than those exemplified herein.

I claim:
 1. A method for preparing asphalt and polymer compositionscomprising: (a) heating a mixture of asphalt and an elastomeric polymerto within an optimum crosslinking temperature range; and (b) adding acrosslinker to the mixture, where the crosslinker is selected from thegroup consisting of a sulfur-containing derivative and elemental sulfurand mixtures thereof. where the optimum crosslinking temperature rangeis that where the resulting asphalt/polymer composition has a differencebetween the top and bottom softening points of 20° C. or less.
 2. Themethod of claim 1 where in heating the mixture, the elastomeric polymeris a vinyl aromatic/conjugated diene elastomer.
 3. The method of claim 2where the elastomeric polymer is a styrene-butadiene copolymer.
 4. Themethod of claim 1 where in adding the crosslinker, the crosslinker isselected from the group consisting of elemental sulfur,mercaptobenzothiazole (MBT), thiurams, and mixtures thereof.
 5. Themethod of claim 4 where in adding the crosslinker, the crosslinkercomprises a thiuram polysulfide.
 6. The method of claim 5 where thethiuram polysulfide has the formula:

where R¹ and R² are the same or different alkyl substitutents havingfrom 1 to 4 carbon atoms, and wherein M is a metal selected from zinc,barium or copper, and n is 0 or 1, and where the optimum crosslinkingtemperature range is above 180° C.
 7. The method of claim 6 where theoptimum crosslinking temperature range is between about 185 and about190° C.
 8. The method of claim 1 where the elastomeric polymer comprisesfrom about 1 to 20 wt % of the asphalt/polymer mixture and where theasphalt has a penetration of between about 20 and 320 dmm.
 9. The methodof claim 1 where the crosslinker is present in an amount ranging fromabout 0.01 to 0.1 wt %, based on the weight of the asphalt/polymermixture.
 10. A method for preparing asphalt and polymer compositionscomprising: (a) heating a mixture of asphalt and an elastomeric polymerto within an optimum crosslinking temperature range; and (b) adding athiuram polysulfide crosslinker to the mixture, where the crosslinker isselected from the group consisting of a sulfur-containing derivative andelemental sulfur and mixtures thereof. where the optimum crosslinkingtemperature range is that where the resulting asphalt/polymercomposition has a difference between the top and bottom softening pointsof 20° C. or less, and where the range is above 180° C.
 11. The methodof claim 10 where in heating the mixture, the elastomeric polymer is avinyl aromatic/conjugated diene elastomer, and where the crosslinker ispresent in an amount ranging from about 0.01 to 0.1 wt %, based on theweight of the asphalt/polymer mixture.
 12. An asphalt and polymercomposition prepared by the method comprising: (a) heating a mixture ofasphalt and an elastomeric polymer to within an optimum crosslinkingtemperature range; and (b) adding a crosslinker to the mixture, wherethe crosslinker is selected from the group consisting of asulfur-containing derivative and elemental sulfur and mixtures thereof.where the optimum crosslinking temperature range is that where theresulting asphalt/polymer composition has a difference between the topand bottom softening points of 20° C. or less.
 13. The composition ofclaim 12 where in the method, in heating the mixture, the elastomericpolymer is a vinyl aromatic/conjugated diene elastomer.
 14. Thecomposition of claim 13 where in the method, the elastomeric polymer isa styrene-butadiene copolymer.
 15. The composition of claim 12 where inthe method, in adding the crosslinker, the crosslinker is selected fromthe group consisting of elemental sulfur, mercaptobenzothiazole (MBT),thiurams, and mixtures thereof.
 16. The composition of claim 15 where inthe method, in adding the crosslinker, the crosslinker comprises athiuram polysulfide.
 17. The composition of claim 16 where in themethod, the thiuram polysulfide has the formula:

where R¹ and R² are the same or different alkyl substituents having from1 to 4 carbon atoms, and wherein M is a metal selected from zinc, bariumor copper, and n is 0 or 1, and where the optimum crosslinkingtemperature range is above 180° C.
 18. The composition of claim 17 wherein the method, the optimum crosslinking temperature range is betweenabout 185 and about 190° C.
 19. The composition of claim 12 where in themethod, the elastomeric polymer comprises from about 1 to 20 wt % of theasphalt/polymer mixture and where the asphalt has a penetration ofbetween about 20 and 320 dmm.
 20. The composition of claim 12 where inthe method, the crosslinker is present in an amount ranging from about0.01 to 0.1 wt %, based on the weight of the asphalt/polymer mixture.